Friday Illusion: Ghostly images change shape

Think hard: you can transform a circle into a hexagon using the power of your mind. New animations created by Hiroyuki Ito from Kyushu University show how staring at coloured shapes can produce an afterimage that varies in form as well as hue. “This is the first study to show systematic shape changes in after-images involving shape processing mechanisms in the brain,” says Ito.

The first version of the illusion uses solid, stationary shapes. After focusing on yellow circles, blue hexagons typically appear and vice versa. The same effect also occurs with outlines of the shapes.

In another variation, hexagons and circles rotate. Although the movement paints circular shapes on the retina, the animations produce the same shape-changing effect as the static illusions.

After-images are thought to occur when three types of retinal cells are overstimulated. Due to signals lingering on the retina, a blurred version in complimentary colours typically arises after the image disappears. But in this case, the additional shape change could be caused by tiring out areas in the cortex that detect corners and curves.